Modem communication systems employing digital systems for providing voice communications, unlike many analog systems, are required to quantify speech objects for transmission and reception. Techniques of Vector Quantization are commonly used to send voice parameters by sending the index representing a finite number of parameters, which reduces the effective bandwidth required to communicate. The reduction of bandwidth is especially attractive on bandwidth constrained channels. Vector quantization is the process of grouping source outputs together and encoding them as a single block. The block of source values can be viewed as a vector, hence the name vector quantization. The input source vector is then compared to a set of reference vectors called a codebook. The vector that minimizes some suitable distortion measure is selected as the quantized vector. The rate reduction occurs as the result of sending the codebook index instead of the quantized reference vector over the channel. The vector quantization of speech parameters has been a widely studied topic in current research. At low rate of quantization, efficient quantization of the parameters using as few bits as possible is essential. Using suitable codebook structure, both the memory and computational complexity can be reduced. However when bit-errors occur within the transmitted vector, an incorrect decoded vector is received resulting in audible distortion in the re-constructed speech. For example, a channel limited to only 3 kHz currently requires very low bit-rates in order to maintain intelligible speech.
FIG. 1 displays a sentence of speech that has been synthesized using Mixed Excitation Linear Prediction (MELP, MIL-STD-3005) at 2400 bps where the gain parameters of MELP have been quantized over four consecutive frames of speech using Vector Quantization. This technique of vector quantization can be applied to the vocoder (voice coder) model parameters in an attempt to reduce the vocoder's bit-rate required to send the signal over a bandwidth-constrained channel. In this case a VQ codebook of MELP's gain parameters was created using the LBG algorithm (Y. Linde, A. Buzo, and R. M. Gray. An algorithm for vector quantizer design. IEEE Trans. Comm., COM-28:84-95, January 1980) the content of which is hereby incorporated by reference. The parameter values being quantized represent the root mean square (RMS) value of the desired signal over portions of a frame of speech. Two gain values G1 and G2 are computed and range from 10 dB to 77 dB. These gain values are estimated from the input speech signal and quantized. As part of the standard, G2 is quantized to five bits using a 32-level uniform quantizer from 10.0 to 77.0 dB. The quantizer index is the transmitted codeword. G1 is quantized to 3 bits using an adaptive algorithm specified in MIL-STD-3005. Therefore, eight bits are used in the MELP standard to quantize gain values G1 and G2.
FIG. 1 illustrates the effect of quantizing the gain values over four frames using a codebook with 2048 vectors of length eight (four consecutive frames of G1 and G2 values). Four frames of gain codeword (4*8=32) bits have been reduced to an 11-bit codebook index by vector quantization. The resulting VQ gain codebook speech cannot be discerned as being different from the uniform quantizer method that is used in the MELP speech model.
The codebook created with the LBG codebook design algorithm results in an ordering that is dependent on the training data and choices made to seed the initial conditions. The gain codebook order that was trained using the LBG algorithm was further randomized using the random function available in the C programming language. FIG. 2 shows the effect of a 10% Gaussian bit-error rate on the codebook index values sent over the channel. The segment of signal representing silence in FIG. 1 now shows signs of voiced signal in FIG. 2 representing noticeable audible distortion. The signal envelope or shape has also been severely degraded as a result of the channel-errors and the resulting speech is very difficult to understand.
Thus there is a need to improve the bit-error tolerance performance of low-rate vocoders that use Vector Quantization (VQ) in order to reduce the effective bit-rate necessary to send intelligible speech over a bandwidth constrained channel. Likewise, as codebooks increase in size, it becomes a difficult computational task to order the codebooks using current computer techniques, thus there is a need to reduce the computational complexity of ordering codebooks to improve bit-error tolerance performance.
Therefore it is an object of the disclosed subject matter to overcome these and other problems in the art and present a novel system and method for improving the bit-error tolerance of vector quantization codebooks when using a parametric speech model over a bandwidth constrained channel.
It is also an object of the disclosed subject matter to present a novel method to overcome the computational load of a complete solution of locating the optimal codebook ordering that maps vectors with similar Euclidean distance with vector indices with similar Hamming distance. The invention results in a technique that allows ordering of large codebooks such that the distortion of single and many double bit-errors resulting in vectors that have less audible distortion as compared to random ordering.
It is further an object of the disclosed subject matter to present a novel method for improving bit error tolerance of vector quantization codebooks. Embodiments include sorting the codebook vectors based on Euclidian distance from the origin thereby creating an ordered set of codebook vectors and assigning codewords to the codebook vectors in order of their hamming weight and value. A first distortion sum is calculated for all possible single bit errors and a first pair of successive codewords are swapped, and a second distortion sum for all possible single bit errors is calculated. Embodiments of the disclosed subject matter maintain the swapped vectors if the second distortion sum is less than the first distortion sum; thereby creating an improved bit error tolerance codebook.
It is still another object of the disclosed subject matter to present a novel method of transmitting intelligible speech over a bandwidth constrained channel. An embodiment of the method relates quantized vectors of speech to code words, where the quantized vectors approximate in Euclidean distance are assigned to code words approximate in hamming distance; thereby creating an index. Embodiments also encode the speech object by quantizing the speech object and selecting its corresponding codeword from the index and transmitting the codeword over the bandwidth constrained channel for decoding by a receiver using the same index, thereby allowing the transmission of intelligible speech over the bandwidth constrained channel.
Is yet another object of the disclosed subject matter to present a system for vector quantization reordering an LBG codebook to enable communication over bandwidth constrained channels. Embodiments of the system include a processor operably connected to an electronic memory and hard disk drive storage, the hard disk storage containing a computation program; wherein the processor reorders the LBG code book by reassigning quantized vectors close in Euclidian distance to indices close in hamming distance. Embodiments also include an input device operably connected to the hard drive for entering the LBG codebook; and an output operably connected to the processor for storing the reordered codebook.
It is an additional object of the disclosed subject matter to present a novel improvement for a method in a communication system operating over a bandwidth constrained communication channel, of transmitting quantized vectors by transmitting indices corresponding to the quantized vectors. Embodiments of the improvement comprises the step of corresponding quantized vectors close in Euclidean distance to indices close in hamming distance.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal or the claims, the appended drawings, and the following detailed description of the preferred embodiments.